Superfluid Helium Gyroscope Could Measure Earth Dragging Spacetime in a Lab, Physicists Propose
A Berkeley, Vienna and Caltech team says a chilled loop of helium-4 could rival space-based measurements of frame-dragging, on the ground.
Einstein's general theory of relativity says that a spinning object does not just sit in spacetime. It drags spacetime around with it. A team of physicists now proposes a way to measure that tiny effect from a laboratory on the ground.
The effect is called frame-dragging. Earth's rotation slightly pulls the surrounding spacetime along, which slowly changes the direction in which nearby gyroscopes point relative to distant stars. The phenomenon is backed by real-world observations, but the signal is extraordinarily small.
Researchers at the University of California, Berkeley, the Austrian Academy of Sciences and the California Institute of Technology describe a possible experiment in Physical Review Letters. It would use helium-4 cooled until it becomes a superfluid, a liquid that flows without friction. That happens below 2.17 kelvin, about minus 271 degrees Celsius.
The proposed instrument is a gyrometer, a device that measures rotation. The team would fill a doughnut-shaped tube with superfluid helium and place a barrier with tiny holes, called nanopores, across the loop to form a Josephson junction. When the device rotates, a quantum mechanical phase difference appears across the junction. The researchers would read it out by tracking the motion of a mechanical diaphragm connected to the superfluid loop.
Similar sensors already exist. A group at UC Berkeley led by professor Richard Packard has used them to measure Earth's rotation. The difficulty is that frame-dragging adds a minuscule extra phase, and heat can swamp it. "The additional phase deriving from the Earth's frame-dragging effect is extremely small and its contribution can be overwhelmed by random phases due to thermal noise," said first author Kai-Isaak Ellers.
So the team analyzed the thermal noise of the proposed device in a new way, guided by Caltech experimentalist Keith Schwab's experience with superfluids. Their conclusion: cooling far below the superfluid transition could cut the noise enough to matter. "A superfluid gyrometer operating at, for example, 10 mK, has the potential to match or even exceed the sensitivity of the best frame-dragging measurements to date, with significantly less measurement time," Ellers said. He noted a practical edge: "It could be performed in a laboratory here on Earth, rather than in space."
The project grew out of a sabbatical visit by Berkeley's Birgitta Whaley and Ellers to Markus Aspelmeyer's group at the Institute for Quantum Optics and Quantum Information in Vienna, where they met Marios Christodoulou, who wanted new uses for superfluid helium in precision gravity experiments. Ellers, Whaley and Christodoulou are theorists, so they brought in Schwab, who builds superfluid devices, to keep the proposal grounded in reality.
The researchers also calculated that changes in the helium could reveal tiny differences in how much time passes along different paths around the loop. That could offer another way to test relativity, including processes on time scales only a few orders of magnitude longer than the Planck time, which is thought to matter in quantum gravity.
The idea is still a proposal. To build the sensor, the team must first develop Josephson junctions for superfluid helium that work at these temperatures and noise levels. If they succeed, it would let physicists watch a purely relativistic effect, one Newtonian gravity does not predict, inside a macroscopic quantum system.
